1. Field of the Invention
This invention relates to a gain control circuit of a video signal reproducing apparatus and particularly to a gain control circuit suitable for use in an automatic color control circuit of a color video signal reproducing apparatus.
2. Description of the Prior Art
There has hitherto been proposed a color video tape recording having, for example, two rotary magnetic heads by means of which every field of a color video signal is recorded in such a way that each magnetic tracks corresponds to one field, and the tracks are formed alternately by first one of the heads and then the other on a magnetic tape. Each track extends obliquely to the longitudinal direction of the tape. During playback, every field of a color video signal is reproduced by these heads, alternatively, from the magnetic track of each field. In a color video tape recorder of this kind, the level of the chrominance signal in the reproduced signal is sometimes diferent between these heads due to irregular characteristics of the heads, unequal characteristics of amplifying systems supplied with signals from the individual heads, uneven contact with the tape, or the like. When a color video signal having such a chrominance signal with the variable level is applied to a television receiver for producing a picture, an undesirable color flicker is produced in the reproduced picture.
It has already been known to provide a variable gain circuit in the transmitting path of reproduced chrominance signals and to control the gain of the circuit according to the level of burst signals separated from the chrominance signals to make the levels of the reproduced chrominance signals constant.
It has also previously been proposed to obtain a constant level of chrominance signals by applying a burst signal to an amplitude detector circuit, such as a synchronous detector circuit, to derive a detected output which is applied to a hold circuit.
The output signal of this hold circuit is then applied to the variable gain circuit as a gain control signal. In this case, the level variation of the chrominance signal during each field from each rotary head is removed, but another drawback is produced. The level of the chrominance signal in each field is made constant but when the level is different for every rotary head, the output signals of the hold circuit corresponding to each rotary head provide slanting rise and decay portions at the beginning and end of each field. As a result, the reproduced chrominance signals are varied by the variable gain circuit in such a manner that the level during every field is made constant except at the beginning and end of each of the fields. This is caused by the fact that since a burst signal is provided at every horizontal period, the response to variable gain cannot be speeded up and, in practice, is further delayed due to noise or the like.